Option #1: Crushed stone or cobblestone over a well-drained base. The need for a drainage base will vary with the local ground conditions, e.g. sandy vs clay.
Option #2: lay 4" thick solid concrete blocks on a bed of sand, either just short strips set where the wheels will rest or the entire pad.
I used #2 for a couple of trailers and 3 different motorhomes, moving the bed of blocks to accommodate the wheel position for each new rig.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC